The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
A semiconductor device, particularly, a semiconductor integrated circuit is manufactured generally by use of a lead frame.
FIG. 6 illustrates a typical configuration of the lead frame. This lead frame 100 includes an external frame 101 for ensuring a strength of the whole structure, a die pad (which may be called as an “island”) connected to this external frame 101 via connecting members known as tie bars 102 and provided substantially at the center, and a plurality of leads 104 disposed along the periphery of the tie pad 103. Inner ends, facing to the die pad 103, of the plurality of leads 104 are separated from the die pad 103, and connected to the external frame 101 via connecting members referred to as dam bars 105 at the middles of the respective leads. The outer lead segments from the dam bars 105 are exposed from a mold sealed by a resin as will be explained later. These exposed lead segments are called outer leads 104a, while the inner lead segments from the dam bars 105 are sealed by the resin and referred to as inner leads 104b. 
Materials used for the lead frame are nickel alloys, Kovar and so on, however, copper or copper alloys composed mainly of copper have been major materials over the recent years.
A semiconductor chip is fixed to the die pad 103 of the thus constructed lead frame 100 by use of a conductive bonding agent such as a silver paste etc. and a solder ribbon etc. Then, electrodes on this semiconductor chip are connected the inner ends of the leads via wires by wire-bonding, and the whole of these portions is sealed by the resin. After cutting off the dam bars 105 remaining outwardly of the resin mold body, the outer leads 104a protruding from the resin mold body undergo forming such as bending and so on. Further, normally solder plating etc is effected in order to enhance a connectivity when connecting the outer leads, thereby obtaining a semiconductor device as a complete product.
This lead frame requires an excellent connectivity to the wire and an excellent connectivity of the outer leads to a printed wiring board etc.
What deteriorates the connection characteristic thereto is an oxide film formed due to oxidation of the lead frame itself. This oxide film appears on the surface of the lead frame due to a heating history in the process of producing and preserving the material and in the process of an assembly of the semiconductor. Copper is preferable in terms of its electric characteristic but has a weak point of its being easy to be oxidized. Therefore, a rustproof film is normally provided on the surface immediately after manufacturing the lead frame by punch-out, etching and so forth in order to prevent the oxide film from being formed thereon. This rustproof film normally involves the use of a benzotriazole (BTA) series.
Further, as described above, the inner leads are connected via the wires to the semiconductor chip. The following is requirements for this wire, wherein the wire is not corroded over a long period of time, exhibits an excellent extendability, and is easy to work and to effect ball-up (ball-shaping) in the atmospheric air. A gold wire as a material meeting these requirements is normally used.
Further, the portions, undergoing the wire-bonding, of the inner leads are normally plated with a noble metal such as silver, palladium and so on in order to enhance a reliability of the connections between these wires and the inner leads.
The film composed of the benzotriazole-series rustproof agent hitherto coated on the lead frame in the process of manufacturing the lead frame for preventing the oxidation, has high heat resisting temperature, and it is therefore required that a complete decomposition be attained by applying the heat at a temperature of 350° C. or higher in the die-bonding process in order to ensure preferable die-bonding and wire-bonding characteristics. The wire-bonding is irresistible against such a higher temperature, and hence the benzotriazole-series rustproof agent can be applied to only the semiconductor devices adopting a high-temperature die-bonding method of applying the heat at the temperature of 350° C. or higher in the die-bonding process, which is, for instance, a solder die-bonding method carried out at an atmospheric temperature in the vicinity of 400° C. Note that this rustproof agent contains a variety of components, some of which are decomposed at the later 200′s° C., and therefore the oxide film is partially formed and will be detached in the later processes, with the result that this might adversely affect quality of manufactured devices. Further, the rustproof agent is decomposed by undergoing the heat history in the assembling process to generate a residual substance, and therefore brings about a decline of adhesion of the resin to the lead frame in the resin molding process, resulting in lowering the reliability of the semiconductor device.
On the other hand, a demand for reducing the costs for the semiconductor device is accelerated day by day, however, a proportion of the costs for the noble metals in the semiconductor device as described above is extremely large and becomes an obstacle against reducing the total costs. For example, gold used as the wire material adopted in the wire-bonding process is expensive and largely fluctuates in the market price, and, the majority of the gold wire purchase price being a cost for the (unprocessed) metal, the scheme of reducing the costs is very close to the limit.
An endeavor for discarding other noble metals is underway, however, substitute materials and manufacturing methods not using the previous metals have their own demerits, and there is nothing determinative.
Furthermore, the silver paste used for the die-bonding involves the use of silver as a noble metal of which the price is high and might fluctuate as an undesired factor, and the solder ribbon contains lead. The decision of specifying lead as a harmful substance in terms of the environmental problem is underway, and it is desirable that lead should not be used. None of materials superior to solder in terms of the price, the characteristics and the productivity is not yet discovered at the present, though a replacement with materials such as silver, bismuth (Bi) etc is examined.
Moreover, the outer lead is normally subjected to Sn-Pb solder plating and palladium (Pd) plating. As explained above, the decision that lead contained in the solder plating is specified as the harmful substance is underway, while palladium is the expensive noble metal, and it is desirable that the use of both of lead and palladium be avoided.